A sector phased array acoustic imaging system using phase changing means, heterodyning, tap selectors and a summing delay line is described in U.S. Pat. No. 4,140,022 issued Feb. 20, 1979 to Samuel H. Maslak entitled ACOUSTIC IMAGING APPARATUS. The phase changing means of that patented imaging system, including heterodyning means, connects between pairs of adjacent transducer elements and selected taps on a master summing delay line. The delay line taps are just close enough together to provide reasonable overlap of the pulse envelopes of the electrical signals received from individual transducer elements. Dynamic focusing is attained by adjusting the phases of the signals throughout reception using heterodyning means so that the cycles of carrier signal within the overlapped pulses have reasonable phase coherence, as is described in the patent.
However, that patented system cannot operate in a linear scanning mode, in which all acoustic scan lines are substantially parallel, without the addition of multiplexer means. That system could operate in the linear scanning mode in a known manner with the inclusion of a programmable analog multiplexer which is M.times.M in further conjunction with transducer switching means as described in U.S. Pat. No. 4,224,829. Construction of a M.times.M analog multiplexer is expensive, since M is typically 64 or so for a high performance system, and is complex hardware since full bandwidth and dynamic range of the low-level transducer outputs must be preserved.